Avoiding mri-interference with co-existing systems

ABSTRACT

MRI interference with a co-existing treatment system may be reduced or avoided by carrying out RF-sensitive operations of the treatment system only when gradient field activity of the MRI system is suppressed.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. patent application Ser. No. 13/222,086,filed on Aug. 31, 2011, the entire disclosure of which is herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates, generally, to medical diagnosis andtreatment methods guided by magnetic resonance imaging (MRI), and, morespecifically, to approaches to minimizing MRI-induced interferences.

BACKGROUND

Magnetic resonance imaging may be used in conjunction with ultrasoundfocusing in a variety of medical applications. Ultrasound penetrateswell through soft tissues and, due to its short wavelengths, can befocused to spots with dimensions of a few millimeters. As a consequenceof these properties, ultrasound can be and has been used for variousdiagnostic and therapeutic medical purposes, including ultrasoundimaging and non-invasive surgery. For example, focused ultrasound may beused to ablate diseased (e.g., cancerous) tissue without causingsignificant damage to surrounding healthy tissue. An ultrasound focusingsystem generally utilizes an acoustic transducer surface, or an array oftransducer surfaces, to generate an ultrasound beam. In transducerarrays, the individual surfaces, or “elements,” are typicallyindividually controllable, i.e., their vibration phases and/oramplitudes can be set independently of one another, allowing the beam tobe steered in a desired direction and focused at a desired distance. Theultrasound system often also includes receiving elements, integratedinto the transducer array or provided in form of a separate detector,that help monitor the focused ultrasound treatment, primarily for safetypurposes. For example, the receiving elements may serve to detectultrasound reflected off interfaces between the transducer and thetarget tissue, which may result from air bubbles on the skin that needto be removed to avoid skin burns. The receiving elements may also beused to detect cavitation in overheated tissues (i.e., the formation ofcavities due to the collapse of bubbles formed in the liquid of thetissue).

To visualize the target tissue and guide the ultrasound focus duringtherapy, magnetic resonance imaging may be used. In brief, MRI involvesplacing a subject, such as the patient, into a homogeneous staticmagnetic field, thus aligning the spins of hydrogen nuclei in thetissue. Then, by applying a radio-frequency (RF) electromagnetic pulseof the right frequency (the “resonance frequency”), the spins may beflipped, temporarily destroying the alignment and inducing a responsesignal. Different tissues produce different response signals, resultingin a contrast among theses tissues in MR images. Because the resonancefrequency and the frequency of the response signal depend on themagnetic field strength, the origin and frequency of the response signalcan be controlled by superposing magnetic gradient fields onto thehomogeneous field to render the field strength dependent on position. Byusing time-variable gradient fields, MRI “scans” of the tissue can beobtained. Many MRI protocols utilize time-dependent gradients in two orthree mutually perpendicular directions. The relative strengths andtiming of the gradient fields and RF pulses are specified in a pulsesequence and may illustrated in a pulse sequence diagram.

Time-dependent magnetic field gradients may be exploited, in combinationwith the tissue dependence of the MRI response signal, to visualize, forexample, a brain tumor, and determine its location relative to thepatient's skull. An ultrasound transducer system, such as an array oftransducers attached to a housing, may then be placed on the patient'shead. The ultrasound transducer may include MR tracking coils or othermarkers that enable determining its position and orientation relative tothe target tissue in the MR image. Based on computations of the requiredtransducer element phases and amplitudes, the transducer array is thendriven so as to focus ultrasound into the tumor. Alternatively oradditionally, the ultrasound focus itself may be visualized, using atechnique such as thermal MRI or acoustic resonance force imaging(ARFI), and such measurement of the focus location may be used to adjustthe focus position. These methods are generally referred to asmagnetic-resonance-guided focusing of ultrasound (MRgFUS).

The simultaneous operation of ultrasound and MRI apparatus can lead toundesired interferences. For example, MRI is very sensitive toradio-frequency (RF) noise generated by the focused ultrasound system(see, e.g., U.S. Pat. No. 6,735,461). Conversely, focused ultrasoundprocedures often involve RF-sensitive operations (such as the ultrasounddetection that may accompany treatment with focused ultrasound) that areeasily disturbed by RF excitation signals and/or time-varying fieldgradient generated by the MRI system. Prior-art approaches to avoidingsuch interference include shielding as well as signal filtering and/orprocessing. Shielding the ultrasound system from interfering MR signalstypically requires covering or surrounding the whole transducer andassociated cables in metallic shield. In some systems, however, acousticconstraints prevent complete encapsulation of the ultrasound-receivingelements, resulting in penetration of, e.g., the front layer of areceiver and/or the cables by some amount of RF noise. Filteringunwanted RF disturbances from desired RF signals requires sophisticatedelectronics that is often difficult to implement and might damage thewanted signal. Digital signal processing usually increases the systemcomplexity significantly, and is sometimes insufficient to eliminate allinterferences. Accordingly, there is a need for alternative approachesin MRgFUS applications to minimize or avoid interferences between thetwo systems.

SUMMARY

Embodiments of the present invention reduce or eliminate MRIinterference with a co-existing system by exploiting MRI pulse sequences(also called “MRI recipes”) that include periods when the MRI gradientsare relatively inactive (or “quiet”). The co-existing system may be atreatment system such as, for example, an ultrasound imaging probe orphased-array ultrasound transducer system. The operating procedure ofthe co-existing system may be synchronized with the MRI recipe such thatRF-sensitive operations are carried out only during time intervals whenthe MRI gradients are inactive (and which are typically also free of MRexcitation or response signals). Inactive gradients include gradientsthat are substantially zero, and may further include non-zero, buttemporarily constant (or “static”) gradients. In practice, gradients arecharacterized as inactive if the RF noise that they generate is below apredetermined maximum acceptable noise limit, which generally depends onthe particular application.

Avoidance of MRI-caused interference with ultrasound operations inaccordance herewith is advantageous in that it generally eliminates (orat least reduces) the need for shielding, filtering, or digital signalprocessing of RF signals. Various embodiments of the present inventionavoid the drawbacks of the prior art by confining the RF-receivingperiods of the ultrasound system to time intervals in which there is nointerference from MRI that would have to be shielded, filtered, orremoved by post-processing. As a result, however, the total imaging ortreatment time may be slightly increased. Therefore, it may be desirablefor certain applications to combine the synchronization of RF-sensitiveultrasound operations and MRI gradient idle times with shielding,filtering, and/or signal processing to optimize the overalleffectiveness of the MRgFUS system.

In a first aspect, the invention provides a method of performingtreatment of an anatomic region in conjunction with MR imaging of theregion, where the treatment includes at least one RF-sensitiveoperation. The RF-sensitive operation may be, for example, an ultrasoundoperation, which may include or consist of a cavitation oracoustic-reflection measurement or ultrasound imaging. The methodinvolves temporarily suppressing gradient field activity during an MRimaging operation, and carrying out the RF-sensitive operation only whenthe gradient field activity is suppressed. Non-RF-sensitive treatmentoperations may be carried out while the gradient fields are active.

In some embodiments, gradient-field-activity suppression corresponds tosubstantially constant gradient fields, i.e., gradient fields whosemagnitude changes by less than a predetermined fraction or absolutevalue. For example, in certain embodiments, gradient fields are deemed“substantially constant” if their magnitude changes, at a given point intime, by less than 0.1% of their maximum change rate.

The method may further include signaling onset of thegradient-field-activity suppression by an MRI apparatus (e.g., to anapparatus performing the treatment). In some embodiments, the MR imagingconforms to a pulse sequence that specifies the onset time of thegradient-field-activity suppression; the RF-sensitive operation maybegin based on this onset time. During the pulse sequence, the gradientfield activity may be suppressed periodically. The pulse sequence mayhave an associated repetition time period. The method may includedetermining the end of such repetition time period, carrying out theRF-sensitive operation after the repetition time period has ended, andtriggering a new repetition time period after completion of theRF-sensitive operation. In some embodiments, the method includessynchronizing the treatment and the MR imaging with a synchronizationsignal. Alternatively or additionally, the treatment and the MR imagingmay be synchronized to a common clock.

In another aspect, the invention provides a system for performingtreatment of an anatomic region in conjunction with MR imaging of theregion, where the treatment includes at least one RF-sensitiveoperation. The system includes an MRI apparatus for imaging the anatomicregion (which involves gradient field activity), and a treatmentcontroller (e.g., a controller associated with or part of the treatmentsystem) in communication with the MRI apparatus. The treatmentcontroller causes the RF-sensitive operation to be carried out only whenthe gradient field activity is suppressed. The system may furtherinclude an MRI controller for operating the MRI apparatus in accordancewith a pulse sequence. In some embodiments, the MRI controller signalstime intervals of the pulse sequence where the gradient field activityis suppressed to the treatment controller, such that the RF-sensitiveoperation is only performed during these time intervals In someembodiments, the treatment controller causes performance of theRF-sensitive operation when the pulse sequence ends, and triggersrepetition of the pulse sequence after completion of the RF-sensitiveoperation. The system may further include the treatment apparatus (whichmay be, e.g., an ultrasound transducer) that performs the treatment.

In yet another aspect, a controller for synchronizing an MRI apparatuswith a treatment system (such as an ultrasound system) is provided. Thecontroller includes a module for receiving information about an MRIpulse sequence specifying time intervals wherein gradient fields aresuppressed, and a module for initiating the RF-sensitive ultrasoundoperation at the onset of the gradient-field suppression based on theinformation.

A further aspect of the invention is directed to an MRI system operablein conjunction with a treatment system for performing MR imaging of ananatomic region in conjunction with treatment of the region (whichincludes one ore more RF-sensitive operations). The MRI system includesan MRI apparatus for imaging the anatomic region and an MRI controller.The MRI controller operates the MRI apparatus in accordance with a pulsesequence that includes time intervals of gradient field activity as wellas time intervals where the gradient field activity is suppressed. Thecontroller signals the time intervals where the gradient field activityis suppressed to the treatment apparatus so as to cause performance ofthe RF-sensitive operation during these time intervals.

Another aspect is directed to a treatment system operable in conjunctionwith an MRI system for performing treatment (including RF-sensitiveoperations) of an anatomic region in conjunction with MR imaging of theregion. The system includes a treatment apparatus (such as, orincluding, an ultrasound transducer) for performing the treatment, andtreatment controller for causing performance of the RF-sensitiveoperation in response to an end of an MRI pulse sequence comprisinggradient field activity, and triggering repetition of the pulse sequenceafter completion of the RF-sensitive operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be more readily understood from the followingdetailed description, in particular, when taken in conjunction with thedrawings, in which:

FIG. 1 is a schematic drawing of an MRgFUS system in accordance with oneembodiment;

FIG. 2 is a perspective view of an MRgFUS system in accordance with oneembodiment;

FIGS. 3A-3C are schematic drawings illustrating the interaction betweenan MRI apparatus and an ultrasound transducer in accordance with variousembodiments;

FIG. 4 is a pulse sequence diagram illustrating an exemplary MRIprotocol as well as synchronization-signal and ultrasound-detectionperiods in accordance with one embodiment;

FIG. 5A is a spectrum of an MRI interference signals detected by theultrasound receiver in the absence of synchronization; and

FIG. 5B is a spectrum of an MRI interference signal detected by theultrasound receiver after synchronization in accordance with oneembodiment.

DETAILED DESCRIPTION

FIGS. 1 and 2 illustrate an exemplary MRgFUS system 100 in whichsynchronization of ultrasound and MRI procedures may advantageously bepracticed. As shown in FIG. 1, the system 100 includes a plurality ofultrasound transducer elements 102, which are arranged in an array 103at the surface of a housing 104. The array may comprise a single row ora matrix of transducer elements 102. In alternative embodiments, thetransducer elements 102 may be arranged without coordination, i.e., theyneed not be spaced regularly or arranged in a regular pattern. The arraymay have a curved (e.g., spherical or parabolic) shape, as illustrated,or may include one or more planar or otherwise shaped sections. Itsdimensions may vary, depending on the application, between millimetersand tens of centimeters. The transducer elements 102 may bepiezoelectric ceramic elements. Piezo-composite materials, or generallyany materials capable of converting electrical energy to acousticenergy, may also be used. To damp the mechanical coupling between theelements 102, they may be mounted on the housing using silicone rubberor any other suitable damping material.

The transducer elements 102 are separately controllable, i.e., they areeach capable of emitting ultrasound waves at amplitudes and/or phasesthat are independent of the amplitudes and/or phases of the othertransducers. A transducer controller 106 serves to drive the transducerelements 102. For n transducer elements, the controller 106 may containn control circuits each comprising an amplifier and a phase delaycircuit, each control circuit driving one of the transducer elements.The controller 106 may split an RF input signal, typically in the rangefrom 0.1 MHz to 4 MHz, into n channels for the n control circuit. It maybe configured to drive the individual transducer elements 102 of thearray at the same frequency, but at different phases and differentamplitudes so that they collectively produce a focused ultrasound beam.The transducer controller 106 desirably provides computationalfunctionality, which may be implemented in software, hardware, firmware,hardwiring, or any combination thereof, to compute the required phasesand amplitudes for a desired focus location. In general, the controller106 may include several separable apparatus, such as a frequencygenerator, a beamformer containing the amplifier and phase delaycircuitry, and a computer (e.g., a general-purpose computer) performingthe computations and communicating the phases and amplitudes for theindividual transducer elements 102 to the beamformer. Such systems arereadily available or can be implemented without undue experimentation.

The system 100 further includes an MRI apparatus 108 for imaging thetarget tissue and/or ultrasound focus. To aid in determining therelative position of transducer array and MRI apparatus 108, thetransducer array may have MR trackers 110 associated with it, arrangedat a fixed position and orientation relative to the array. The trackers110 may, for example, be incorporated into or attached to the housing104. If the relative positions and orientations of the MR trackers 110and transducers 102 are known, MR scans of the MR trackers 110implicitly reveal the transducer location in MRI coordinates, i.e., inthe coordinate system of the MRI apparatus 108. The transducercontroller 106, which receives MRI data containing the MR trackerlocation, can then set the phases and amplitudes of the transducers 102to generate a focus 112 at a desired location or within a desired targetregion. In some embodiments, a user interface 114 in communication withthe transducer controller 106 and/or the MRI apparatus 108 facilitatesthe selection of the focus location or region in MR coordinates.

The system 100 generally also has the capability to detect ultrasound,which serves to monitor the application of ultrasound for safetypurposes. For example, ultrasound reflections off tissue interfaces thatintersect the ultrasound beam path may be analyzed to ensure, ifnecessary by adjustment of the treatment protocol, that such interfacesare not inadvertently overheated. Further, measurements of the receivedcavitation spectrum may be used to detect cavitation resulting from theinteraction of ultrasound energy with water-containing tissue. Inaddition, the visualization of the tissue and target may be supplementedby ultrasound imaging, for example, to facilitate tracking a movingtarget. Ultrasound detection may be accomplished with the ultrasoundtransducer array 103. For example, treatment and imaging periods may beinterleaved, or a contiguous portion of the array 103 or discontiguoussubset of transducer elements 102 may be dedicated to imaging while theremainder of the array 103 focuses ultrasound for treatment purposes.Alternatively, a separate ultrasound receiver 116, which may be, e.g., asimple ultrasound probe or array of elements, may be provided. Theseparate receiver 116 may be placed in the vicinity of the ultrasoundtransducer array 103, or even integrated into its housing 104. Ifsynchronization in accordance herewith is not utilized, the ultrasoundreceiver 116 needs to be shielded, e.g., by a surrounding conductivestructure serving as a Faraday cage, to be at least partially effective.

FIG. 2 illustrates the MRI apparatus 108 in more detail. The apparatus108 may include a cylindrical electromagnet 204, which generates therequisite static magnetic field within a bore 206 of the electromagnet204. During medical procedures, a patient is placed inside the bore 206on a movable support table 208. A region of interest 210 within thepatient (e.g., the patient's head) may be positioned within an imagingregion 212 wherein the electromagnet 204 generates a substantiallyhomogeneous field. A set of cylindrical magnet field gradient coils 213may also be provided within the bore 206 and surrounding the patient.The gradient coils 213 generate magnetic field gradients ofpredetermined magnitudes, at predetermined times, and in three mutuallyorthogonal directions. With the field gradients, different spatiallocations can be associated with different precession frequencies,thereby giving an MR image its spatial resolution. An RF transmittercoil 214 surrounding the imaging region 212 emits RF pulses into theimaging region 212, and receives MR response signals emitted from theregion of interest 210. (Alternatively, separate MR transmitter andreceiver coils may be used.)

The MRI apparatus 108 generally includes an MRI controller 216 thatcontrols the pulse sequence, i.e., the relative timing and strengths ofthe magnetic field gradients and the RF excitation pulses and responsedetection periods. The MRI controller 216 may be combined with thetransducer controller 106 into an integrated system control facility.The MR response signals are amplified, conditioned, and digitized intoraw data using an image processing system, and further transformed intoarrays of image data by methods known to those of ordinary skill in theart. Based on the image data, a treatment region (e.g., a tumor) isidentified. The image processing system may be part of the MRIcontroller 216, or may be a separate device (e.g., a general-purposecomputer containing image processing software) in communication with theMRI controller 216 and/or the transducer controller 106. An ultrasoundphased array 220, disposed within the bore 206 of the MRI apparatus and,in some embodiments, within the imaging region 212, is then driven so asto focus ultrasound into the treatment region. The drive signals arebased on the MRI images, which provide information about the positionand orientation of the transducer surface(s) with respect to the MRIapparatus and/or the focus location. To monitor the ultrasoundtreatment, an ultrasound receiver 222 may also be disposed within thebore 206 of the MRI apparatus.

FIGS. 3A-3C schematically illustrates the interaction between an MRIapparatus 300 and a focused ultrasound system 310 in accordance withvarious embodiments of the invention. The MRI apparatus 300 includes RFtransmitter coils 320, and gradient coils 322 for generatingtime-varying magnetic gradients across the tissue to be imaged. Bothtransmitter-coil and gradient-coil emissions fall in the RF range andcan potentially disturb focused ultrasound procedures. The MRItransmitter coils 320 generate electromagnetic pulses with frequenciesin the range from about 50 MHz to about 150 MHz to induce spin flipping.The gradients generated by the gradient coils 322 are typically updatedat kHz or MHz frequencies, and are substantially constant betweensuccessive updates. For example, the gradient value (i.e., the magneticfield strength of the gradient field) may be controlled digitally at asampling rate of 250kHz by applying a new voltage every fourmicroseconds. These small control steps generate RF noise, mainly at thesampling frequency (i.e., 250 kHz in the example) and its harmonics(i.e., 500 kHz, 750 kHz, etc.). A step in the gradient value is usuallyimplemented by a controlled ramp whose slope is proportional to thevoltage step. The resulting RF noise is generally proportional to thevoltage step as well. However, even during nominally static gradients,control steps exist and resulting in some level of RF noise (althoughsignificantly less than is generated during ramps). In other words,non-zero static gradients are quieter than dynamic gradients, but arenot completely quiet.

The MRI apparatus 300 includes a database 324 (stored, e.g., on a harddrive of a computer, which may be the same computer as is used for MRimage processing) for storing pulse sequence diagrams (PSDs). Anassociated sequence controller 326 within the MRI controller 216operates the MRI apparatus in accordance with the specified pulsesequences. As illustrated in FIG. 3A, the sequence controller 326 mayprovide a synchronization signal to the ultrasound control module 328,signaling the onset of gradient idle times, i.e., time intervals inwhich magnetic field gradients, or time variations thereof, arecompletely or partially suppressed. The ultrasound controller module 328may be implemented in the transducer controller 106, and initiateRF-sensitive ultrasound operations only during the gradient idle times.

Ultrasound operations that are particularly sensitive to RF disturbancesfrom the MRI apparatus 300 include ultrasound imaging (in parallel withMRI) and measurements of the cavitation spectrum or of acousticreflections, all of which generally have low signal voltages associatedwith them (e.g., voltages in the mV range and below). During thesemeasurements, the ultrasound receiver 330 (which may be the transduceroperated in “listening” mode, or a separate, dedicated receiver device)converts the acoustic signals into electrical RF signals. Such signalscan also be created by the RF disturbances from the MRI apparatus 300,resulting in unwanted signal components. Since the detected signalsgenerally have lower power than, e.g., focused ultrasound ablationpulses, they are particularly sensitive to such perturbations.

FIG. 4 shows a PSD illustrating, for a typical MR gradient echo pulsesequence, the relative timing of the RF excitation pulse, the magneticfield gradients in three directions, and the MR response signal, whichoccurs at the echo time (TE). The sequence may be periodically repeated;the period is denoted as the repetition time TR and may be, for example,in the range from 20 to 30 ms. FIG. 4 further shows the timing of thesynchronization signal relative to the MRI sequence, as well as theperiod during which RF-sensitive ultrasound operations (i.e., generally,ultrasound detection) may be carried out, which may last, for example, 1ms. Ultrasound operations that are not especially sensitive to RFdisturbances may be carried out at any time, including periods duringwhich the MRI gradients are active. In fact, focused ultrasoundapplication times are often in the range from about 10 to about 30seconds. Thus, ultrasound application may begin long before and end longafter the MRI sequence. In embodiments in which the ultrasoundtransducer is alternately used for focused ultrasound application andRF-sensitive ultrasound detection, the non-RF-sensitive operations arepreferably carried out during active-gradient periods, reserving thegradient idle times for RF-sensitive operations.

In the PSD shown in FIG. 4, gradient idle time is added to eachrepetition time period by design (in a first step of a firstembodiment). During the idle time, the MRI apparatus 300 sends asynchronization signal to the ultrasound system 310 (in a second step ofa first embodiment), which then performs spectrum measurements and otherRF-sensitive operations (in a third step of a first embodiment).Synchronization between MRI and ultrasound detection is, thus,internally controlled by the MRI recipe. In alternative embodiments,synchronization may be effected through control mechanisms external tothe MRI recipe. For example, as illustrated in FIG. 3B, the ultrasoundcontroller module 328 may control the timing of RF-sensitive operationsbased on measurements of RF signals originating from the MRI apparatus300, which may be performed, for example, by the ultrasound receiver 330or by a separate, dedicated RF-noise receiver 332 in communication withthe module 328. The MRI sequence may stop running after each repetitionperiod (either automatically or based on an external control signal) (ina first step of a second embodiment), and the focused ultrasound systemmay identify the end of a repetition period (e.g., by measuring RFsignals generated by the MRI apparatus) (in a second step of a secondembodiment), perform the ultrasound measurements (in a third step of asecond embodiment), and then send a trigger command to the MRI apparatusto resume the MRI sequence, i.e., proceed to the next repetition period(in a fourth step of a second embodiment). The MRI sequence may beinterrupted after each repetition time, or after a multiple of therepetition time (such that one or more repetition periods are skippedbefore the next ultrasound measurement is carried out). The system mayalso be programmed to perform ultrasound measurements only after certainMRI procedures, for example, only after thermal imaging sequences.External control generally provides a high degree of flexibility intiming MR imaging and RF-sensitive ultrasound operations, therebyfacilitating time efficiency in the overall procedure.

The synchronization of the MRI and focused ultrasound apparatus 300, 310may be modified in additional ways. For example, the sequence controller326 and ultrasound controller module 328 may be integrated into a singlecontrol module that sends synchronization or clock signalssimultaneously to both apparatus 300, 310, or controls the MRItransmitter coils 320, gradient coils 322, and ultrasound receiver 330directly. Alternatively, as shown in FIG. 3C, a separate controller 340may communicate with conventional MRI and ultrasound apparatus that eachinclude their individual controllers. The controller 340 may include afirst module 342 that determines when gradient-field activity issuppressed, e.g., based on information it receives about an MRI pulsesequence specifying time intervals during which the gradients are quiet.The module 342 may also send control signals to the sequence controller326 to stop MRI operation at the end of a sequence. The first module 342may communicate gradient idle time to a second module 344 responsiblefor initiating the RF-sensitive treatment operation.

In general, functionality for synchronizing an MRI apparatus and afocused ultrasound system as described above, whether integrated withthe MRI and/or ultrasound controller or provided by a separatecontroller, may be structured in one or more modules implemented inhardware, software, or a combination of both. For embodiments in whichthe functions are provided as one or more software programs, theprograms may be written in any of a number of high level languages suchas FORTRAN, PASCAL, JAVA, C, C++, C#, BASIC, various scriptinglanguages, and/or HTML. Additionally, the software can be implemented inan assembly language directed to the microprocessor resident on a targetcomputer; for example, the software may be implemented in Intel 80x86assembly language if it is configured to run on an IBM PC or PC clone.The software may be embodied on an article of manufacture including, butnot limited to, a floppy disk, a jump drive, a hard disk, an opticaldisk, a magnetic tape, a PROM, an EPROM, EEPROM, field-programmable gatearray, or CD-ROM. Embodiments using hardware circuitry may beimplemented using, for example, one or more FPGA, CPLD or ASICprocessors.

FIGS. 5A and 5B show the spectra of MRI interference signals detected bythe ultrasound transducer with and without synchronization of thedetection period with gradient idle times. As illustrated,synchronization can reduce MRI disturbances by about an order ofmagnitude. Note that the signals in FIGS. 5A and 5B are free ofcavitation effects. FIG. 5A shows a signal corrupted by gradient noise,while FIG. 5B, shows a clean signal that contains only background noise.

In some embodiments, the synchronization methods described above areused in conjunction with shielding, signal filtering, and/or processing.This allows RF-sensitive operations to be carried out during portions ofMR sequences in which the gradients are sufficiently inactive. Forexample, if synchronization is combined with shielding, there isgenerally a trade-off between the amount of shielding used and themaximum acceptable noise. The less shielding is used, the quieter thegradients need to be to avoid undesired interference between the MRIsystem and the ultrasound (or other co-existing) system. Noisereductions due to shielding depend on the particular material used(e.g., iron, copper, or nickel) as well as on the frequency range ofinterest, and can readily be ascertained based on graphs and tabulationsof absorption and reflection coefficients that are available in theliterature. For example, at frequencies of around 1 MHz, a 3 mm thickiron shield reduces the noise by about 100 dB. For a given maximumacceptable noise level (which, in turn, depends on the signal filteringand processing capabilities of the system), the maximum allowablegradients can be computed based on the noise reduction achieved byshielding.

Although the present invention has been described with reference to anultrasound transducer system and other specific details, it is notintended that such details should be regarded as limitations upon thescope of the invention. For example, systems and methods forsynchronizing MR imaging with treatment modalities other than focusedultrasound therapy that include RF-sensitive operations are alsoincluded within the scope of the invention. Moreover, it is to beunderstood that the features of the various embodiments described hereinare not necessarily mutually exclusive and can exist in variouscombinations and permutations, even if such combinations or permutationsare not made express herein, without departing from the spirit and scopeof the invention. In fact, variations, modifications, and otherimplementations of what is described herein will occur to those ofordinary skill in the art without departing from the spirit and thescope of the invention.

What is claimed is:
 1. A system for performing treatment of an anatomicregion in conjunction with MR imaging of the region, the treatmentcomprising at least one RF-sensitive operation, the system comprising:an MRI apparatus for imaging the anatomic region, the imaging comprisinggradient field activity; and a treatment system comprising a treatmentcontroller in communication with the MRI apparatus, the controller beingconfigured to: (i) cause the MRI apparatus to temporarily suppress thegradient field activity during imaging of the anatomic region; (ii)receive a signal indicative of gradient-field-activity suppression; and(iii) initiate the RF-sensitive operation in response to the receivedsignal indicative of the gradient-field-activity suppression.
 2. Thesystem of claim 1, further comprising an MRI controller for operatingthe MRI apparatus in accordance with a pulse sequence that includes timeintervals where the gradient field activity is suppressed, the signalbeing indicative of the time intervals.
 3. The system of claim 1,further comprising an MRI controller for operating the MRI apparatus inaccordance with a pulse sequence, the signal being indicative of an endof the pulse sequence.
 4. The system of claim 3, wherein the treatmentcontroller is further configured to trigger repetition of the pulsesequence so as to allow the MRI controller to operate the MRI apparatusin accordance therewith after completion of the RF-sensitive operation.5. The system of claim 1, further comprising a measurement system formeasuring RF signals originating from the MRI apparatus, wherein thecontroller is further configured to perform the RF-sensitive operationbased on the measured RF signals.
 6. The system of claim 1, wherein theMRI apparatus comprises an MRI controller for transmitting the signalindicative of gradient-field-activity suppression to the treatmentcontroller.
 7. The system of claim 1, wherein the treatment systemfurther comprises at least one of an ultrasound transducer, a cavitationdetection device, or a reflection detection device, in communicationwith the treatment controller, for performing the RF-sensitiveoperation.
 8. The system of claim 1, wherein the treatment controller isfurther configured to cause the gradient field activity to beperiodically temporarily suppressed.
 9. The system of claim 1, furthercomprising a system clock for synchronizing the RF-sensitive operationand imaging of the anatomic region.
 10. A method of performing treatmentof an anatomic region in conjunction with magnetic resonance (MR)imaging of the region, the treatment comprising at least oneradio-frequency-sensitive (RF-sensitive) operation, the methodcomprising the steps of: (i) during an MR imaging operation, temporarilysuppressing gradient field activity; and (ii) in response to a signalindicative of gradient-field-activity suppression, initiating theRF-sensitive operation.
 11. The method of claim 10, wherein thegradient-field-activity suppression corresponds to zero gradient fields.12. The method of claim 10, wherein the gradient-field-activitysuppression corresponds to constant gradient fields.
 13. The method ofclaim 10, wherein the signal indicative of gradient-field-activitysuppression is transmitted from an MR imaging apparatus performing theMR imaging operation to an RF device performing the RF-sensitiveoperation.
 14. The method of claim 10, wherein the MR imaging conformsto a pulse sequence specifying an onset time of thegradient-field-activity suppression, and the signal is indicative of theonset time.
 15. The method of claim 14, wherein the gradient fieldactivity is suppressed periodically during the pulse sequence.
 16. Themethod of claim 10, wherein the MR imaging conforms to a pulse sequencehaving a repetition time period associated therewith, the signalindicating when the repetition time period ends.
 17. The method of claim16, further comprising triggering a new repetition time period aftercompletion of the RF-sensitive operation.
 18. The method of claim 10,wherein the signal is a synchronization signal specifying relativetiming between the RF-sensitive operation and the MR imaging.
 19. Themethod of claim 10, wherein the RF-sensitive operation and the MRimaging are synchronized to a common clock.
 20. The method of claim 10,wherein the RF-sensitive operation is an ultrasound-detection operation.21. The method of claim 20, wherein the ultrasound-detection operationcomprises at least one of measuring cavitation, measuring acousticreflections, or ultrasound imaging.
 22. A controller for synchronizingan MRI apparatus performing an MR imaging operation with a treatmentsystem performing operations at least one of which is RF-sensitive, thecontroller comprising: a first module for (i) receiving informationabout an MRI pulse sequence specifying time intervals wherein gradientfield activity is suppressed, and (ii) based thereon, determining whenthe gradient-field activity is suppressed; and a second module forinitiating the RF-sensitive operation at an onset of thegradient-field-activity suppression based on the determination.
 23. AnMRI system operable in conjunction with a treatment system forperforming MR imaging of an anatomic region in conjunction withtreatment of the region, the treatment comprising at least oneRF-sensitive operation, the MRI system comprising: an MRI apparatus forimaging the anatomic region, the imaging comprising gradient fieldactivity; a database for storing a pulse sequence comprising timeintervals where the gradient field activity is suppressed; and an MRIcontroller configured to: (i) operate the MRI apparatus in accordancewith the pulse sequence, and (ii) signal the time intervals to thetreatment system so as to cause performance of the RF-sensitiveoperation during the time intervals.
 24. A treatment system operable inconjunction with an MRI system for performing treatment of an anatomicregion in conjunction with MR imaging of the region, the treatmentcomprising at least one RF-sensitive operation, the treatment systemcomprising: a treatment apparatus for performing the treatment; and atreatment controller configured to: (i) receive a signal indicative ofan end of an MRI pulse sequence comprising gradient field activity, (ii)in response to the received signal, initiate performance of theRF-sensitive operation; and (iii) trigger repetition of the pulsesequence after completion of the RF-sensitive operation.
 25. Thetreatment system of claim 24, wherein the treatment apparatus comprisesan ultrasound transducer.